Why has it become common for rank to be worn on a kind of hanger from the front of battle dress? Obviously shoulder straps have disappeared - because they got in the way? But why the hanger? Why not a (Velcro'd) patch? And why has it become so widespread relatively quickly?

From my understanding, many modern types of body armor and pack systems completely cover shoulder and collar tabs, so many countries moved rank to the chest. Some armies do use velcro tabs, but I've heard that many troops were concerned that the loud, ripping sound of velcro being removed would ruin the element of surprise in the field. Apparently these velcro tabs could be easily torn off when changing gear, low crawling, entering and exiting a vehicle, etc.

As for the becoming widespread so quickly...there are only a handful of nations that extensively fund research of uniforms and gear. When there is a breakthrough in technology or new innovation, it is much easier for other nations to copy it rather than spend hundreds of thousands or even millions field testing new designs.